The junction formed by the interface of two dissimilar semiconductors, which may have either the same or opposite conductivity types, is commonly termed a "heterojunction" and is useful in many types of devices. For example, the use of a heterojunction as an emitter base junction in a bipolar transistor was proposed by Schottky. Somewhat later, Kroemer proposed the use of a heterojunction as a wide bandgap emitter in a bipolar transistor. Many device uses for heterojunctions have been proposed since Kroemer's work including, for example, light sources, such as lasers and light emitting diodes, and photodetectors.
It is widely appreciated by the skilled artisan that useful devices may have more than one heterojunction. For example, double heterojunction lasers having an active layer sandwiched between two cladding layers are now widely used. Additionally, devices may have a plurality of thin layers of a first semiconductor composition interleaved with a plurality of thin layers of a second semiconductor composition. The resulting interleaved structure is commonly termed a "superlattice," and one type of superlattice structure is called a "quantum well laser." Such a structure arises when the active layer is of the order of the de Broglie wavelength, and two-dimensional quantization of the carrier energy levels occurs.
Heterojunctions between many different types of semiconductors have been fabricated and studied. For example, heterojunctions between AlGaAs and GaAs have been fabricated, with a second layer grown on a first layer, and are useful in many types of devices. This type of heterojunction is relatively easy to fabricate because as one varies the composition from pure GaAs to pure AlAs, the lattice constant of the semiconductor material does not vary very much. That is, for Al.sub.x Ga.sub.1-x As, the lattice constant is relatively constant as x varies. As a consequence, crystal perfection of the second layer is relatively easy to obtain. Matching of lattice constants, however, is not sufficient to ensure growth of good quality heterojunctions. For example, GaP and Si are approximately lattice matched, but GaP/Si heterojunctions are difficult to grow because these semiconductors may dope each other. Consequently, it is difficult to control the doping.
However, for many other materials systems of potential interest, the lattice constant of the second semiconductor layer differs significantly from that of the first semiconductor layer, and growth of a high-quality second layer is often difficult to obtain.
An example of a heterostructure having layers with significantly different lattice constants results from the growth of Ge or Ge.sub.x Si.sub.1-x, x greater than 0.0, layers on Si substrates or vice versa, that is, Si or Ge.sub.x Si.sub.1-x, x less than 1.0, layers on Ge substrates. Such heterostructures are not easy to fabricate because the lattice constants may differ by as much as 4 percent. While this constitutes a large lattice mismatch, it is less than that of such systems as silicon/sapphire in which the lattice constants differ by approximately 10 percent. Accordingly, much effort has been directed toward devising methods which would permit the growth of Ge/Si heterojunctions and more generally, the growth of Ge.sub.x Si.sub.1-x films on Si for values of x greater than 0.0 and less than or equal to 1.0. However, no artisan has yet succeeded in growing smooth, defect-free, germanium-rich Ge.sub.x Si.sub.1-x layers epitaxially on Si.
For example, Thin Solid Films, 22, pp. 221-229, 1974, describes the growth of germanium films on silicon substrates. The germanium films had what the authors termed a "fragmentary structure," which could be more accurately termed "cracked." High-quality films were not obtained. The authors used a relatively poor vacuum, 10.sup.-6 Torr, and apparently little attention was devoted to substrate preparation prior to growth.
Another paper is in Applied Physics, 8, pp. 199-205, 1975, in which the growth of one-dimensional GeSi superlattices having periods varying between 100 and 800 Angstroms is described. The films contained only relatively small and varying amounts of germanium, that is, a multilayer structure comprising layers of Ge.sub.x Si.sub.1-x was grown, and x never exceeded 0.15. Growth was at a substrate temperature of 750 degress C. Because of the lattice mismatch between the epitaxial layers having varying amounts of Ge, misfit dislocations were observed, and it was found that the number of such dislocations depended upon the thickness of the GeSi layers. It was further found that this relationship was in qualitative agreement with the theory of van de Merwe.
The effective growth conditions for GeSi films and, in particular, the cooling rate after film deposition, are discussed in Thin Solid Films, 30, pp. 91-98, 1975. A large number of fragmentary structures, that is, cracked layers, was observed, and the quality of the structures depended upon the cooling rate. A relatively poor vacuum was used as the authors apparently believed that variations in the vacuum quality between 10.sup.-6 Torr and 10.sup.-9 Torr had no effect upon layer quality.
The relationship between the elastic strain and misfit dislocation density in Ge.sub.0.08 Si.sub.0.92 films on Si substrates is discussed in Thin Solid Films, 44, pp. 357-370, 1977. The authors grew films having only a small amount of Ge, that is, x equal to 0.08 at a substrate temperature of 750 degrees C., and found that the film strain was mainly compressive due to the lattice mismatch. They further found that below a critical film thickness there were no misfit dislocations, and above this critical thickness there were misfit dislocations which were generated to release the strain generated. The critical film thickness was 0.1 .mu.m. The authors again concluded that their measured dependence of misfit dislocation distance on film thickness agreed with theoretical predictions. They also concluded that the critical film thickness was larger than predicted and that the films were not in an equilibrium state. Although the films were grown under ultra-high vacuum conditions, substrate cleaning and quality were apparently believed not critical.
A further study of imperfections in the Ge.sub.x Si.sub.1-x films is described in Thin Solid Films, 55, pp. 229-234, 1978. A solid solution of Ge.sub.x Si.sub.1-x was deposited at a substrate temperature between 800 and 1000 degrees C. The value of x varied uniformly through the layer thickness, from a value of 0.35 to a value of 0.95. Micrographs of the surface revealed numerous dislocations.
Growth of pure Ge films on Si substrates is described in Applied Physics Letters, 38, pp. 779-781, May 15, 1981, and Applied Physics Letters, 41, pp. 1070-1072, Dec. 1, 1982. The former paper reported smooth growth at substrate growth temperatures below 550 degrees C. and rough growth at higher temperatures. The latter paper reported epitaxial growth at temperatures within the range between 375 and 425 degrees C. Both papers were directed toward the preparation of materials for solar cells and, in both papers, the films reported had high dislocation densities.